Hydrostatic transmissions have many uses, including the propelling of vehicles, such as mowing machines, and offer a stepless control of the machine's speed. A typical hydrostatic transmission system includes a variable displacement main hydraulic pump connected in a closed hydraulic circuit with a fixed displacement hydraulic motor. The closed hydraulic circuit includes a first conduit connecting the main pump outlet with the motor inlet and a second conduit connecting the motor outlet with a pump inlet. Either of these conduits may be the high pressure line depending upon the direction of pump displacement from neutral. For most applications, the pump is driven by a prime mover, such as an internal combustion engine or an electrical motor, at a certain speed in a certain direction. Changing the displacement of the main pump will change its output flow rate, which controls the speed of the motor. Pump outflow can be reversed, thus reversing the direction of the motor. In a vehicle, the motor is typically connected through suitable gearing to the vehicle's wheels or tracks.
Hydrostatic transmissions generate heat as the hydraulic fluid is circulated between the pump and the motor. Friction between moving parts of the pump and/or motor also generates heat. Hydrostatic transmissions commonly operate at temperatures in excess of 275 degrees Fahrenheit. In order to operate at such high temperatures, an expensive synthetic oil, such as MOBILE 1, is often used. Operating at such high temperatures raises various concerns such as durability.
External oil coolers have been used to dissipate heat from hydrostatic transmission. Such external oil coolers are typically connected to the closed loop hydraulic circuit and circulate hot oil from the closed loop through a heat exchanger located remotely from the transmission. Cooled fluid from the exchanger is then returned to the closed loop. Prior art external oil coolers mounted to a vehicle separate from the hydrostatic transmission are connected to the hydraulic transmission via supply and return lines. Such external coolers can be expensive and require additional space onboard a vehicle for mounting, thereby increasing the overall size of the hydrostatic transmission.
Another approach to cooling hydrostatic transmissions involves mounting a fan to a rotatably driven shaft of the hydrostatic transmission. The fan is configured to circulate air around the external surfaces of the hydrostatic transmission to dissipate heat. This approach, though relatively inexpensive, is limited to heat transfer through the external surface of the hydrostatic transmission which can be inadequate, particularly as hydrostatic transmissions continue to become more compact and powerful.
Still another approach is described in U.S. Pat. No. 5,622,051. According to this approach, the hot oil is passed through a cooling tube equipped with cooling fins for dissipation of heat. The cooling tube passes around an output shaft to which a fan is connected, and thus some of the cooling tube is located in the path of the forced air flow. In another embodiment, a transmission housing has formed therein an oil flow passage adjacent an outer wall equipped with cooling fins to dissipate heat from the oil. While affording some cooling, these arrangements are considerably less efficient and effective than the above-noted external coolers.